The present invention relates to a brush structure of a motor, and more particularly to a brush structure of a motor capable of reducing current ripple and torque ripple in a motor-driven power steering device for vehicles using a DC brush motor, and an effective technology when applied in a motor-driven power steering device using the same motor.
As the technology studied by the present inventor, for example, a motor-driven power steering device has generally been used as a steering device of turning wheels such as front wheels of an automobile. Hitherto, the motor-driven power steering device is known as rack assist type, in which a DC brush motor is provided coaxially with a rack shaft, and a steering assisting force is obtained by the DC brush motor provided on this rack shaft.
The motor-driven power steering device of rack assist type comprises a DC brush motor, a rack shaft coaxially provided in the DC brush motor, turning wheels coupled with the rack shaft by rack and pinion through a coupling machanism, and a steering wheel, in which the steering assisting force generated by the DC brush motor is transmitted to the rack shaft through a ball screw mechanism. The turning wheels are steered by the steering assisting force due to the DC brush motor and by the manual steering force due to manipulation of the steering wheel, so that the driver""s effort in steering is lessened.
Such motor-driven power steering device using the DC brush motor was investigated by the inventor, and the following facts were disclosed. In this motor-driven power steering device, the current ripple generated when the brush commutates among segments of the commutator, and the resulting torque fluctuations cause working sound, magnetostriction sound, radio noise, and cogging feel during steering operation, which is known to impair the commercial value significantly.
As the countermeasure, hitherto, the brush shape was modified, or the contact state of brushes and segments were stabilized by long-time aging, but such measures were main causes of increasing the cost.
It is hence an object of the invention, by paying attention to current ripple and torque ripple, to present a brush structure of a motor capable of reducing the current ripple and torque ripple substantially while minimizing the cost increase, and a motor-driven power steering device using the same motor capable of suppressing the generation of working sound, magnetostriction sound, radio noise, and cogging feel during steering operation.
A brush structure of a motor of the invention comprises an armature core having a plurality of slots; a plurality of coils wound around each one of the plurality of slots; a commutator having a plurality of segments connected to each one of the plurality of coils; a pair of first brushes electrically contacting with each segment of the commutator for feeding power to each coil from each segment; and a pair of second brushes disposed at a phase angle for inverting the brush contacting position to each segment corresponding to the first brushes, and electrically contacting with each segment of the commutator for feeding power to each coil from each segment. Therefore, each phase of current waveforms of the first brushes and second brushes is inverted to cancel each other.
A motor-driven power steering device of the invention uses this motor as a DC brush motor for generating a steering assisting force, and turning wheels are steered by the steering assisting force due to the DC brush motor and by a manual steering force due to manipulating a steering wheel. In particular, the invention is preferably applied in a motor-driven power steering device using a motor of high response.